A fuel cell can convert chemical energy to electrical energy by promoting a chemical reaction between two reactant gases.
One type of fuel cell includes a cathode flow field plate, an anode flow field plate, a membrane electrode assembly disposed between the cathode flow field plate and the anode flow field plate, and two gas diffusion layers disposed between the cathode flow field plate and the anode flow field plate. A fuel cell can also include one or more coolant flow field plates disposed adjacent the exterior of the anode flow field plate and/or the exterior of the cathode flow field plate.
Each flow field plate has an inlet region, an outlet region and open-faced channels connecting the inlet region to the outlet region and providing a way for distributing the reactant gases to the membrane electrode assembly.
The membrane electrode assembly usually includes a solid electrolyte (e.g., a proton exchange membrane) between a first catalyst and a second catalyst. One gas diffusion layer is between the first catalyst and the anode flow field plate, and the other gas diffusion layer is between the second catalyst and the cathode flow field plate.
During operation of the fuel cell, one of the reactant gases (the anode reactant gas) enters the anode flow field plate at the inlet region of the anode flow field plate and flows through the channels of the anode flow field plate toward the outlet region of the anode flow field plate. The other reactant gas (the cathode reactant gas) enters the cathode flow field plate at the inlet region of the cathode flow field plate and flows through the channels of the cathode flow field plate toward the cathode flow field plate outlet region.
As the anode reactant gas flows through the channels of the anode flow field plate, the anode reactant gas passes through the anode gas diffusion layer and interacts with the anode catalyst. Similarly, as the cathode reactant gas flows through the channels of the cathode flow field plate, the cathode reactant gas passes through the cathode gas diffusion layer and interacts with the cathode catalyst.
The anode catalyst interacts with the anode reactant gas to catalyze the conversion of the anode reactant gas to reaction intermediates. The reaction intermediates include ions and electrons. The cathode catalyst interacts with the cathode reactant gas and the reaction intermediates to catalyze the conversion of the cathode reactant gas to the chemical product of the fuel cell reaction.
The chemical product of the fuel cell reaction flows through a gas diffusion layer to the channels of a flow field plate (e.g., the cathode flow field plate). The chemical product then flows along the channels of the flow field plate toward the outlet region of the flow field plate.
The electrolyte provides a barrier to the flow of the electrons and reactant gases from one side of the membrane electrode assembly to the other side of the membrane electrode assembly. However, the electrolyte allows ionic reaction intermediates to flow from the anode side of the membrane electrode assembly to the cathode side of the membrane electrode assembly.
Therefore, the ionic reaction intermediates can flow from the anode side of the membrane electrode assembly to the cathode side of the membrane electrode assembly without exiting the fuel cell. In contrast, the electrons flow from the anode side of the membrane electrode assembly to the cathode side of the membrane electrode assembly by electrically connecting an external load between the anode flow field plate and the cathode flow field plate. The external load allows the electrons to flow from the anode side of the membrane electrode assembly, through the anode flow field plate, through the load and to the cathode flow field plate.
Because electrons are formed at the anode side of the membrane electrode assembly, that means the anode reactant gas undergoes oxidation during the fuel cell reaction. Because electrons are consumed at the cathode side of the membrane electrode assembly, that means the cathode reactant gas undergoes reduction during the fuel cell reaction.
For example, when hydrogen and oxygen are the reactant gases used in a fuel cell, the hydrogen flows through the anode flow field plate and undergoes oxidation. The oxygen flows through the cathode flow field plate and undergoes reduction. The specific reactions that occur in the fuel cell are represented in equations 1-3. EQU H.sub.2.fwdarw.2H.sup.+ +2e.sup.- (1) EQU 1/2O.sub.2 +2H.sup.+ +2e.sup.-.fwdarw.H.sub.2 O (2) EQU H.sub.2 +1/2O.sub.2.fwdarw.H.sub.2 O (3)
As shown in equation 1, the hydrogen forms protons (H.sup.+) and electrons. The protons flow through the electrolyte to the cathode side of the membrane electrode assembly, and the electrons flow from the anode side of the membrane electrode assembly to the cathode side of the membrane electrode assembly through the external load. As shown in equation 2, the electrons and protons react with the oxygen to form water. Equation 3 shows the overall fuel cell reaction.
In addition to forming chemical products, the fuel cell reaction produces heat. One or more coolant flow field plates are typically used to conduct the heat away from the fuel cell and prevent it from overheating.
Each coolant flow field plate has an inlet region, an outlet region and channels that provide fluid communication between the coolant flow field plate inlet region and the coolant flow field plate outlet region. A coolant (e.g., liquid de-ionized water) at a relatively low temperature enters the coolant flow field plate at the inlet region, flows through the channels of the coolant flow field plate toward the outlet region of the coolant flow field plate, and exits the coolant flow field plate at the outlet region of the coolant flow field plate. As the coolant flows through the channels of the coolant flow field plate, the coolant absorbs heat formed in the fuel cell. When the coolant exits the coolant flow field plate, the heat absorbed by the coolant is removed from the fuel cell.
To increase the electrical energy available, a plurality of fuel cells can be arranged in series to form a fuel cell stack. In a fuel cell stack, one side of a flow field plate functions as the anode flow field plate for one fuel cell while the opposite side of the flow field plate functions as the cathode flow field plate in another fuel cell. This arrangement may be referred to as a bipolar plate. The stack may also include monopolar plates such as, for example, an anode coolant flow field plate having one side that serves as an anode flow field plate and another side that serves as a coolant flow field plate. As an example, the open-faced coolant channels of an anode coolant flow field plate and a cathode coolant flow field plate may be mated to form collective coolant channels to cool the adjacent flow field plates forming fuel cells.
Hydrogen gas typically is produced by a reformer which reforms methanol or other hydrocarbons, such as natural gas, into hydrogen (H.sub.2), carbon dioxide (CO.sub.2), and other byproducts, such as carbon monoxide (CO). As known in the art, reforming methods include steam reforming, catalytic partial oxidation, and autothermal reforming. The reformed fuel stream contains high levels of carbon monoxide (greater than 10,000 ppm) which "poisons" the anode catalyst of the fuel cell by binding to the fuel cell catalyst thereby inhibiting hydrogen fuel from being oxidized. Typically achieving levels that are less than 100 ppm of CO in the reformed fuel stream is necessary to avoid catalyst poisoning of the fuel cell.
In order to reduce the concentration of CO to lower levels, the reformed fuel stream passes through a "shift" reactor and a "PROX" reactor. In a "shift" reactor, steam and most of the CO in the reformed fuel stream react in the presence of a catalyst to produce CO.sub.2 and H.sub.2. The reformed fuel stream exiting the "shift" reactor includes a residual level of CO, typically between about 3,000 to about 10,000 ppm, which is preferentially oxidized by flowing the output of the "shift" reactor through the PROX reactor. The PROX reactor includes a catalyst to promote the preferential oxidation of CO by air (0.sub.2) in the presence of H.sub.2, but without consuming (by oxidizing) large quantities of H.sub.2. The chemical reaction for the PROX reactor is shown in equation (3). EQU CO+1/2O.sub.2.fwdarw.CO.sub.2 (3)
The amount of air required for the PROX reaction typically is about 1.5 to about 4.0 times the stoichiometric amount of equation 1. If the amount of air is increased above this level, large amounts of hydrogen in the reformed fuel stream are consumed by excess 0.sub.2. In general, as the level of CO in the reformed fuel stream changes, the amount of air supplied to the PROX reactor must also change to compensate for either a decrease of or an increase of CO concentration.
To control the amount of air input into the PROX reactor, in one method a CO detector can monitor the level of CO in the fuel stream exiting the PROX reactor and send a signal proportional to the CO level to a computer. Based upon the level of CO, the computer adjusts the air flow into the PROX reactor to reduce the CO concentration to non-poisoning levels.